Wavelength-selective infrared (IR) absorptance of modified complex gratings of heavily doped silicon with nanoscale features is studied by a finite-difference time-domain numerical scheme. The purpose of this work is to demonstrate the possibility of using complex gratings and nanoscale surface features to modify far-field radiative properties. By properly choosing the carrier concentration and geometry, silicon complex gratings exhibit a broadband absorptance peak resulting from the excitation of surface plasmon polaritons. Meanwhile, the absorptance of four modified complex gratings with attached features has been numerically investigated for their impact. First, the first peaks of the absorptance spectra of gratings due to Wood’s anomaly remain unchanged; the second peaks shift toward longer wavelengths in modified complex gratings, as compared with complex gratings without attached features. The modified complex gratings with attached features on both sides of the ridges have the most obvious effect on the absorptance spectral shift. Second, the spectral absorptance curves of complex gratings with square features in three sizes (100, 200, and $300\mathrm{nm}$ ) are compared and show that the peak wavelength shifts toward longer wavelengths with enlarged feature size. These combined effects of doped silicon, complex gratings, and the addition of submicrometer-sized features to grating side walls can be used for further tailoring thermal radiative properties, which may be very useful for enhancing the performance of IR detectors.